The richness and diversity indexes revealed that the microbiota present in GARH and GARN exhibit greater bacterial genera diversity and richness than the microbiota in GARM. This is in agreement with previous studies on R. microplus that were collected from cattle . Previous studies in male and female ticks of Ixodes ovatus, I. persulcatus, and Amblyomma variegatum have shown differentiated microbiome profiles both at the taxonomic and functional levels between sexes of the same tick species .
A metagenomic study showed that the microbiome profile in ticks is related to metabolic processes and that their resilience and adaptability to the environment is related to their sex . In addition, geographical location, temperature, humidity, species, sex, anatomical location, and type of food have been shown to affect the microbiome of ticks [23–28]. In our study, although ticks were of the same species and were collected from the same host, significant differences were found in bacterial diversity and richness related to the sex and developmental stage of ticks.
Among the 147 different genera identified, the core microbiome that included the majority of the most prevalent genera stood out. Several of the identified genera within the core microbiome are known to be human pathogens (i.e., Salmonella, Vibrio, Paracoccus, Staphylococcus, Pseudomonas, Corynebacterium, Cloacibacterium, and Acinetobacter). In addition, a greater bacterial microbiome was shared between nymph and female ticks [14 (9.5%)] compared to that shared between male and female ticks [5 (3.4%)]. We suggest that these differences have a behavioral origin. Thus, female and nymph ticks are more prone to remain on the same host, whose microbiota impact on the tick gut microbiome, while male ticks frequently change hosts . This hypothesis is supported by studies on other genera that reported higher relative abundance and alpha diversity in female ticks than in male ticks .
The most prevalent genus among the three groups of ticks was identified as Salmonella, whose members cause gastrointestinal tract infection and dysentery and can lead to serious clinical conditions, especially in children . The genus Vibrio, the second in abundance (15.6%), represents a finding of great interest as, to the best of our knowledge, this is the first study showing its presence in R. microplus. The genus Vibrio is a common commensal of aquatic arthropods and has a remarkable capacity for adaptation to the environment [30, 31]. Its presence evinces the adaptation of this genus to the gastrointestinal system of R. microplus, which inhabits a jungle ecosystem. Many Vibrio are opportunistic pathogens of both arthropods and humans. Therefore, studying the virulence of the identified species is essential [30–32]. Paracoccus, the third most abundant genus (6.97%), is a coccobacillary bacterium that is typically present in a wide range of ecosystems . Staphylococcus, with a prevalence of 6.63%, is mainly related to infections in soft tissues and has been previously reported in the intestines of R. microplus and with a high prevalence in female Amblyomma variegatum [9, 22]. Pseudomonas showed an abundance of 5.87% in R. microplus. In previous studies, the presence of this bacterial genus in R. microplus and in male Amblyomma variegatum with a high prevalence has been reported [9, 22]. Pseudomonas has been suggested to be involved in the infection of soft tissues, including the tissues of the respiratory system [34, 35]. The presence of Corynebacterium, with an abundance of 5.87%, is important because some Corynebacterium species produce the diphtheria toxin or can cause osteomyelitis . In addition, this genus has been previously identified in eggs and male adults of R. microplus . Cloacibacterium, with a prevalence of 2.93% in R. microplus, are gram-negative bacteria that proliferate in aqueous environments with high content of organic matter . Acinetobacter, with an abundance of 2.53%, has been reported in a metagenomic study in I. persulcatus, I. pavlovskyi, and Dermacentor reticulatus . Sphingomonas, the ninth most abundant genus (2.47%), includes non-fermenting and strictly aerobic gram-negative bacteria. Some species, such as S. paucimobilis and S. wittichii, can cause infections in immunocompromised patients [39,40].
In contrast to the bacterial microbiome relevant to human health identified in our study, a previous study on bacterial diversity in R. microplus collected from cattle identified Ehrlichia sp., Coxiella sp., and Bartonella sp. . This indicates that the bacterial microbiome would also depend on the host parasitized by the ticks. Some bacteria, such as Leptospira interrogans, Mycobacterium, Salmonella, Clostridium, and Pasteurella, and tick genera, such as Haemaphysalis, Dermacentor, and Amblyoma, have been identified in the genus Pecari [42-44]. In our case, R. microplus, a tick that mainly parasitizes cattle , was found in P. tajacu (sajino). P. tajacu was possibly tick infected due to the proximity of Botijón Village, where livestock farming is practiced. This highlights that ticks can infect cattle, P. tajacu, and humans, with the potential risks of pathogen transmission that this implies.
Regarding the role of bacteria in ticks, note that nonpathogenic microorganisms present in ticks could cause infections in humans and other animals. For example, ecological studies have shown that Rickettsia, Francisella, and Coxiella, which are considered vertebrate pathogens, can change their pathogenic role and have a mutualistic and symbiotic relationship with ticks . Therefore, studying the interaction between the bacterial microbiota and ticks is of utmost importance for the control of pathogens and the development of the arthropod . Coxiella sp. infects at least two-third of the ticks and is important for the survival of Amblyomma americanum and Rhipicephalus sp. [46, 47]. Nonetheless, it has not been found in our study. Coxiella sp. and Francisella sp. are linked to the synthesis of vitamins necessary for the survival of ticks [48–50]. Likewise, other symbiotic bacteria, such as Francisella, Rickettsia, and Rickettsiella, have been reported , with Rickettsia sp. and Coxiella sp. having become strict endosymbionts .